Theoretical mechanistic studies on the degradation of alizarin yellow R initiated by hydroxyl radical

Abstract

The degradation of azo dyes has attracted many research efforts not only due to the resulting environmental problems but also because the azo compounds with various substituents may show different degradation mechanism. It has been computationally found here, for the first time, that the HO• initiated cleavages of C-N and N-N bonds of alizarin yellow R with carboxyl group are kinetically competitive. In view of the formation of HO• adducts, the C-N and N-N bond cleavages of the hydrazone tautomer of alizarin yellow R are also kinetically competitive, but the former is more thermodynamically favorable. This result is different from that previously reported for the hydrozone tautomers of Acid Orange 7 and Acid Orange 8 containing hydroxyl and azo groups in neighboring positions, which are favorable to follow C-N bond cleavage mechanism both kinetically and thermodynamically. The decarboxylation occurs via an attack of HO• to the benzene ring carbon connecting to the carboxyl group rather than a direct attack of HO• to the carboxyl carbon atom. The anion form has higher reactivity than the neutral form in all of the reactions investigated. In addition, a water molecule as a proton relay reagent could significantly reduce the energy barrier for the N-N bond cleavage of alizarin yellow R. Figure 2. Optimized structures (distances in Å) and relative energies in the solution for HO• attacking the C3 and C6 sites of AH to result in the cleavage of the C-N bond (energies in kcal/mol relative to the energy sum of AH and HO•) X. JIAO ET AL. wileyonlinelibrary.com/journal/poc a The energies are relative to the energy sum of AH (or A) and HO•. The AH_Ci_TS and AH_Ci_P (i = 1, 2, 17, 18 and 21, atom labeling shown in Chart 3) denote the HO• addition transition states and resulting adducts, respectively. The same is true for the A case. DFT STUDY ON DEGRADATION MECHANISM OF AZO COMPOUNDS

Theoretical investigation on the mechanism of the OH-initiated degradation process of reactive red 2 azo dye

“…They found that for AYRA, the C-N bond cleavage is kinetically competitive with, but thermodynamically more favorable than the N-N bond cleavage. 55 Their conclusion is in line with the corresponding experimental nding, that is, the phenol was observed as major product in the oxidation degradation of AYRA. 56 In the present paper, the dual descriptor result suggests that the attacks by cOH at both the N32 and N33 sites of the azo group, which are associated with the N-N bond cleavage, 55 are not favorite.…”

“…Jiao et al 55 studied the OH-initiated C-N and N-N bond cleavages of alizarin yellow R (AYR, see Scheme 1) and its anion form (AYRA) computationally at the MPW1K/6-311+G(d,p)// B3LYP/6-31G(d) with the integral equation formalism polarized continuum model (IEF-PCM) used in the single point energy calculation. They found that for AYRA, the C-N bond cleavage is kinetically competitive with, but thermodynamically more favorable than the N-N bond cleavage.…”

A DFT Study of Disperse Yellow 119 Degradation Mechanism by Hydroxyl Radical Attack

“…concluded that the reactions of Acid Orange 7 and 8 with •OH radical gave precedence to the C−N bond cleavage mechanism both kinetically and thermodynamically. In contrary, Jiao et al . in their study on neutral and anionic forms of Alizarin Yellow R reported that the C−N and the N−N bond cleavage pathways are kinetically competitive, but the former was found to be more thermodynamically favoured for the anionic form and the hydrazone tautomer only.…”

“…Previous theoretical studies by Ozen et al . and Jiao et al . gave conflicting reports on the degradation mechanisms of azo dyes such as Acid Orange 7, Acid Orange 8 and Alizarin Yellow R, which exist predominantly in hydrazone tautomeric form.…”

A new kind of H-acid monoazo-anthraquinone reactive dyes with surprising colour

“…All calculations in this work were performed at the level of B3LYP/6-31G*, as planted in Gaussion 09 program [18]. Subsequent vibration frequency analyses were preformed for confirmation of the optimized structure as a minimum (N imag = 0).…”